T. C. Chiang | Searcharxiv

arXiv2022

Observation of Pines' Demon in Sr$_2$RuO$_4$

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a "demon," could exist in three-dimensional metals containing more than one species of charge carrier. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral, and do not couple to light, so have never been detected in an equilibrium, three-dimensional metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals, optical properties of metal nanoparticles, "soundarons" in Weyl semimetals, and high temperature superconductivity in, for example, metal hydrides. Here, we present evidence for a demon in Sr$_2$RuO$_4$ from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed of electrons in the $β$ and $γ$ bands, the demon is gapless with a room temperature velocity $v=1.065 \pm 0.12 \times 10^5$ m/s and critical momentum $q_c=0.08$ reciprocal lattice units. Its spectral weight violates low-energy partial sum rules, affirming its neutral character. Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature of multiband metals.

cond-mat.str-el↗

arXiv2013

Emergence of charge density wave domain walls above the superconducting dome in TiSe2

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe2, which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at Pc = 5.1 +/- 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls.

cond-mat.supr-con↗